1. Field of the Invention
The invention relates to write drivers for an inductive head in a magnetic data storage system, and particularly to write drivers designed to operate at high rates of data transfer.
2. Description of Related Art
Conventional storage systems include an inductive head that uses an inductive element to write information on a recording surface of a magnetic media, such as a magnetic disk. The inductive element usually is an inductive coil that writes information by creating a changing magnetic field. A write driver circuit is connected to the inductive at first and second head nodes. During writing operations, the write driver circuit forces a relatively large write current through the inductive coil to create a magnetic field that polarizes adjacent bit positions on a recording surface. Digital information is stored by reversing the polarization of selected bit positions which is done by reversing the direction of the current flow in the inductive head.
The rate at which information can be stored on a recording surface through the inductive head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The rise/fall time of the inductive coil is determined by:
xe2x80x83di/dt=V/L
where di/dt is the rate of change of the write current, V is the available voltage across the inductive coil and L is the head inductance. Therefore the rise time is inversely proportional to the available voltage across the inductive coil.
There is, however, a fundamental limit as to how fast current can change in an inductive head due to its capacitance, parasitic capacitance and write driver capacitance. The combination of inductance and capacitance produces ringing which in conventional high speed write drivers can be controlled by means of a damping resistor coupled across the inductive head. Another limitation on performance of high speed drivers is the current overshoot which occurs in the inductive head element after the current reaches its threshold. The overshoot can cause significant data dependent jitter.
Four U.S. patents are known to deal with magnetic media high speed current drivers.
U.S. Pat. No. 5,386,328 (Chiou, et al.) describes a current mirror based write driver to operate inductive heads for magnetic recording. The write driver has a head voltage that swings between the upper and lower supply rails and thus may be used with CMOS circuits or differential ECL circuits. The 3.3V write driver maintains the same performance characteristics as magnetic recording devices that are powered at supply voltage levels of 5V and 12V.
U.S. Pat. No. 5,822,141 (Chung, et al.) discloses a high speed FET write driver for an inductive head. The FET write driver provides high rate of data transfer to a magnetic storage medium by effectively using the voltage swing provided by the supply voltage. During operation, the low voltage drops across the FET switches allow for a substantial portion of the supply voltage to be available across the inductive head. The circuit is inherently stable and avoids ringing and overshoot which results in improved timing and maintains signal integrity. This patent does not provide a means for controlling current overshoot resulting from the fast rise time of the inductive head write current.
U.S. Pat. No. 5,612,828 (Brannon, et al.) teaches a write driver circuit for driving a magnetic head in an information storage system. The write driver circuit is connected to first and second voltage supply terminals and includes first and second data input terminals for receiving data input signals. The write driver circuit is formed of an H-switch to switch current flow through the magnetic head. A pair of anti-saturation circuits is connected to the H-switch to bias the pull-up transistors of the H-switch
U.S. Pat. No. 5,869,988 (Jusuf, et al.) discloses a high-speed write driver for inductive heads of a magnetic storage medium. The write driver induces a faster write current reversal in an inductive head element by using two significant improvements that operate during the current switch cycle. One is a current boost mechanism to generate a faster rise time when the current switches direction through the inductive head element and the other is a mechanism to reduce the damping resistor effect during the current reversal time.
The present invention describes a high speed write driver for an inductive head of a magnetic storage medium which contains a mechanism to reduce the inductive head current overshoot and therefore reduce jitter and, thus, increase the write cycle frequency. The write driver is comprised of a pair of switches, S1 and S2 coupled to a pair of current sources, I1 and I2. Current source I1 is controlled by first control signal CKNL and current source I2 is controlled by second control signal CKNR which is complementary to CKNL. The first switch S1 is coupled to the first current source I1, and is controlled by first switch control signal CKPL, and the second switch S2 is coupled to the second current source I2 and is controlled by second switch control signal CKPR. In one embodiment, a current booster is coupled to each current source to boost the write current and increase the write current rate of change during current switch transitions; a programmable damping resistor Rd is incorporated in order to momentarily increase Rd and, thus, increase write current and write current rate of change during current switch transitions; switches S1 and S2 which are controlled, respectively, by CKPL and CKPR to maximize rail to rail voltage swing and to suppress current overshoot in the inductive head element by momentarily forcing the voltage across the inductive head element to zero when the threshold current is reached. Other embodiments incorporating the current overshoot suppression with and without the current booster and with or without the programmable resistor are possible.